Semiconductor vertical cavity devices operate by emitting light in the normal direction to an epitaxial growth surface. In such a device a partial or complete cavity is formed on the same semiconductor substrate that includes the active light emitter. Semiconductor vertical cavity devices include vertical-cavity surface-emitting laser (VCSEL) diodes and resonant cavity light emitting diodes (RCLEDs).
A VCSEL is a laser resonator that includes two mirrors which are typically distributed Bragg reflector (DBR) mirrors which have layers with interfaces oriented substantially parallel to the die or wafer surface with an active region including of one or more bulk layers, quantum wells, quantum wires, or quantum dots for the laser light generation in between. The planar DBR-mirrors comprise layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities that may be above 99%.
A laser device typically has a threshold current that is determined by the cavity and gain design. For current levels above this threshold current the device operates as a laser. For currents below this threshold current the device operates in a spontaneous regime. It is also possible a device that emits spontaneous emission is unable to operate as a laser. When the current is above a lasing threshold, the laser generates heat in its light emission process since its quasi-Fermi energy separation for electrons and holes exceeds its emitted photon energy.
A laser also generates some heat when electrons and holes absorb some of the laser light but remain respectively in the conduction band or valence band, known as free carrier absorption. In this case, some of the laser light is absorbed, and this internal absorption also reduces the laser light output. A laser also generates some heat if electrical currents are used to transport electrons and holes to an active region with an external electrical bias. These voltage losses that come from the electron and hole charge transport and the optical absorption that comes from the electron and hole free carrier absorption decrease the laser's efficiency and increase its self-heating. The optical absorption increases the laser's threshold which further decreases its efficiency.
Because the light is most efficiently extracted from a vertical cavity diode's surface in the same region in which electrical current is injected into its active area, a particular problem in semiconductor vertical cavity emitting diodes is the efficient lateral confinement of their electrical current to the same region that confines the optical mode. Known approaches to this problem include the standard intra-cavity oxide approach that suffers from mechanical strain, increases in the vertical cavity diode's drive voltage, and blocking heat flow inside the vertical cavity diode.
Some semiconductor light sources are composite light sources which include an electrically pumped laser within a resonant cavity between a pair of mirrors that optically pumps another laser. The beam from the pump laser is a well directed (i.e., low angular spread) beam because of its cavity and emitter properties from the design of the laser. For this type of composite light source, the composite light source includes a cavity and gain design for the pump laser that brings the pump laser to threshold at a current drive that is less than the threshold of the laser to be pumped. This type of pump laser in a composite light source typically delivers a pump beam of light that has low angular spread due to stimulated emission.